The study will describe the changes in regional blood flow which are elicited following the activation of opioid receptors in the hindbrain. Prior studies had shown that activation of mu or kappa opioid receptors in brain stem nuclei elicited selective increases or decreases in mean arterial pressure and heart rate. The present study will ascertain whether those responses are accompanied by generalized changes in vasomotor function, or by selective increases or decreases in flow to specific vascular beds. This will be accomplished by measuring blood flow in the mesenteric, iliac and renal arteries before and after intraparenchymal injection of opioids having relative selectivity towards mu, delta or kappa opioid receptors, or the endogenous opioids beta- endorphin, dynorphine A, leucine-enkephalin, or methionine- enkephalin. The opioids will be injected into either the nucleus of tractus solitarius or the dorsal motor nucleus of the vagus, two hindbrain nuclei which are important in cardiovascular regulation. The mechanism by which these responses are mediated will be determined by studying the effect of selective alpha- or beta- adrenergic blockade, or by selectively antagonizing vagal postganglionic activity. The results will increase our understanding of the cardiovascular and autonomic responses to central opioid receptor activation by determining how the distribution of cardiac output is modulated by hindbrain opioid receptors. Since opioid systems in the brain appear to help the body respond to conditions of stress, a greater understanding of the cardiovascular and autonomic effects of opioids may lead to innovative therapeutic approaches to cardiovascular disorders which are associated with chronic and acute stress.